[Bug c++/88115] New: Incorrect result from alignof in templates, if also using __alignof__.

[foom at fuhm dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88115

            Bug ID: 88115
           Summary: Incorrect result from alignof in templates, if also
                    using __alignof__.
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: foom at fuhm dot net
  Target Milestone: ---

On both GCC 8.2 and "trunk" on godbolt (https://godbolt.org/z/ykszUZ) given the
following program, compiled for x86 -m32, the final GCCAlignOf static assert
unexpectedly fails.

However, if you comment out the three StdAlignOf lines, the assert starts
passing. Or, if you swap the order such that GCCAlignOf comes first, then it
will pass, and the StdAlignOf assert fails, instead. 

It looks like some caching inside GCC is still treating alignof and __alignof
as the same thing, even though they now (since the fix for PR69560) are
supposed to have differing behavior.

Possibly this is due to both alignof and __alignof mangling to the same
character?

===
static_assert(alignof(double) == 4, "");
static_assert(__alignof__(double) == 8, "");

template<typename _Tp, _Tp __v>
struct integral_constant
{
    static constexpr _Tp value = __v;
};

template <class T>
using StdAlignOf = integral_constant<unsigned long, alignof(T)>;

static_assert(StdAlignOf<double>::value == 4, "");

template <class T>
using GCCAlignOf = integral_constant<unsigned long, __alignof__(T)>;

static_assert(GCCAlignOf<double>::value == 8, "");
===